title for learn-about sections for chitons in A SNAIL'S ODYSSEY
  Ecology
 

Chitons are sluggish and only slowly responsive.  For these reasons their ecology has not excited the same research interest as that of other molluscs. 

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  Habitat preferences and role as herbivores
  Topics of interest in the ecology of chitons include their role as herbivores, considered here, and ORIENTATION & HOMING, GENETIC PATTERNS, and COMPETITIVE INTERACTIONS, considered in other sections.
 
Research study 1
 

A study on herbivores of the red alga Mazzaella splendens in Oregon also includes grazing activity of the black-leather chiton Katharina tunicata.  Removal of all Katharina from intertidal rocky benches leads to re-establishment of Mazzaella in areas where it was previously absent, and removal of limpets Lottia graph showing effect of removal of chiton and other herbivores on algal density on shores in Oregonpelta along with Katharina accentuates this.  Removal of Katharina has a strong positive effect on density of limpets, suggesting photograph of black leather chiton with kelp & a few limpetscompetitive interactions between them.  Gaines 1985 Ecology 66: 473.

NOTE formerly Iridaea cordata

 

 

 





A black leather chiton Katharina tunicata in an area
of kelp Hedophyllum sessile and coralline algae.
Some limpets are present, at least one of which
is Lottia pelta (in the 2 o'clock position) 0.25X

 
Research study 2
 

There have been several studies on effects of chiton herbivory on mediating structure of west-coast intertidal communites.  One of these, conducted in San Juan Island, Washington, concerns the role of herbivory by leather chitons Katharina tunicata1 in affecting competitive interactions between marine plants2.  The authors create 3 types of treatment plots: chiton-removal3, CONTROL, and chiton-addition, before-and-after photographs of a rocky outcropping cleared of black leather chitons over a 1-yr periodand monitor abundance and diversity of the various plants over a 4-yr period.  In the areas where Katharina are removed, algal diversity and abundance increase, leading at the end to a multi-story intertidal kelp bed.  CONTROL sites show year-to-year fluctuations in abundance of the large food-alga Hedophyllum sessile, but otherwise densities remain more-or-less unchanged. 

graph showing effect of removal of chitons Katharina tunicata on density of algaeEffects on several "corticated macrophytes" are shown graphically on the Left and in before-and-after photographs on the Right for a chiton-removal area.  In the Katharina-addition areas, abundance and diversity of algae decline (data not shown here).  In fact, the only plants to thrive under the intense grazing pressure in the chiton-addition areas are surfgrasses Phyllospadix scouleri and diatoms, the former thought possibly to be indigestible by Katharina. Later removal of large kelps from chiton-removal and CONTROL areas lead to rapid increase in abundance and diversity of other algae.  The kelps apparently suppress the growth of smaller forms beneath them.  Because these effects are most pronounced in areas lacking both chitons and kelps, the authors suggest that herbivory and competition have negative, additive effects on the smaller algae.  Duggins & Dethier 1985 Oecologia 67: 183.

NOTE1  in the areas of San Juan Island selected for study, Katharina is the dominant herbivore, reaching densities of up to 100 . m-2 and sizes of 12cm length

NOTE2 other plants, including crusts, diatoms, and surfgrass, are also monitored, but the data are less interesting than those for macroalgae and thus are not included here. The data set presented here represents "corticated macrophytes": Gigartina spp., Fucus spp., Rhodomela larix, Odonthalia spp., Botryoglossum sp., Plocamium sp., Iridaea spp., and other unidentified red species

NOTE3 chitons that wander into the removal areas are cleared out every 4-5mo.  This housekeeping maintains densities in the removal areas at about 8% of original, and in the addition area atabout 180% of original

 
Research study 3
 

histrograms showing effect of removal of chitons Katharina tunicata on abundances of brown kelps  in San Juan Island, Washingtonhistogram showing effect of removal of black leather chitons on kelp growth in Torch Bay, AlaskaA later study by the same authors compares the effects of removing black-leather chitons Katharina tunicata from low intertidal regions in San Juan Island, Washington and Torch Bay, Alaska over a 3-yr period.  Interestingly, while this yields dramatic changes in community structure in San Juan Island, Washington (see histogram on Left), little effect is seen in Alaska. In the Washington experiment the absence of K. tunicata for 3yr results in a kelp bed developing in the low intertidal region consisting of Hedophyllum sessile, Alaria marginata, and Nereocystis luetkeana, and it persists. Smaller algae later increase in abundance and diversity and, after 3yr, a multi-layered algal canopy is present. 

In comparison, effects of removal of chitons on kelps and other algae in Alaska are variable and not related to the presence or absence of Katharina. Note in the graph that while kelps increase in % cover in Alaska following removal of Katharina, they do so in both removal and control plots.  The authors caution that the presence of a strong interaction in one area does not guarantee its presence in another, even if community composition is similar in the 2 regions.  Dethier & Duggins 1988 Mar Ecol Progr Ser 50: 97.

NOTE  chiton densities are similar in the two areas, ranging from 21-57 individuals . m-2

NOTE  replicate sites are cleared in each geographical area, but only results for one replicate are shown here

 
Research study 3.1
 

Each species in an ecological assemblage connects with each other species.  The connections could be competitive interactions for space or food, interactions with predators, or whatever.  Some connections are stronger and more direct than others, such as a herbivore feeding exclusively on a certain seaweed, or they may be weaker and more indirect, such as another herbivore, deprived of food by the first, acting more competitively towards a third species in some manner.  Scientists interested in these and other types of food-web interactions have long been faced with the challenge of how to quantify the strength of each interaction.  An approach that may help solve this is proposed by a researcher at the University of Washington.  The method involves identifying a single strongly competitive species that over time and left unchecked would come to dominate the assemblage.  This path to dominance would represent a baseline against which the effects of manipulation of densities of other species on the timeline could be measured.  A species that significantly disrupts the timeline would be a strong interactor, while one that has minimal effect would be a weak interactor.  The location for the study is the mid-low intertidal area of Tatoosh Island, Washington where the dominant space competitors are 2 species of canopy-forming brown algae.  These provide shelter to numerous fleshy and coralline algae that are eaten by several invertebrate grazers.  The researcher creates replicate “doughnut”-shaped arenas using copper-painted epoxy resin, and monitors survival of the brown-algal sprorelings (the potential dominant space-competitors) at different densities of each consumer species.  The ratio of sporelings surviving after 8mo in treatment plots as compared with control plots divided by grazer density provides an estimate of per capita interaction strength (that is, the capacity to influence the establishment of the brown-algal sporelings) for a particular grazer.  These values are then averaged and can be plotted in ranked order (see graph) to provide comparative data on interaction strengths for each of the grazers.  The author remarks that while a consumer guild would be graph comparing interaction strengths for 7 members of a herbivorous consumer guild on Tatoosh Island, Washingtonexpected overall to show a net negative value, several of the species shown here are actually relatively benign (values around zero).  The interaction strengths of chitons K. tunicata and purple sea-urchins S. purpuratus are strongly negative and statistically indistinguishable, but would not generally compete under normal circumstances because the chitons are competitively inferior and would be driven away or eaten by the urchins.  Although the technique is labour-intensive and would likely need to be assessed seasonally, it provides a first workable method for quantifying per capita interaction strength in ecological communities. Paine 1992 Nature 355: 73; see also Paine 2002 Science 296 (5568): 736.

NOTE  the canopy-forming species are Hedophyllum sessile and Alaria spp., the first dominant in the presence of grazers; the second, dominant in their absence.  The grazers under study include 3 chiton, one sea-urchin, and 3 limpet species

NOTE  copper deters limpets and other organisms from exiting or entering the arena.  Wire barriers similarly constrain sea urchins Strongylocentrotus purpuratus 

Estimated interaction strengths for 7 grazer species, including 3 chitons.
 A value of -1 signifies total annihilation of sporelings, 0 signifies no
effect, and a positive value indicates enhanced survival ofthe sporelings.
The taxonomic status of the limpet Lottia painei is not known

 
Research study 4
 

photograph of study area for chiton-kelp interactionsIn Barkley Sound, British Columbia, black-leather chitons Katharina tunicata inhabit the same intertidal areas as does a potential food and shelter species, the kelp Hedophyllum sessile.

By manipulating densities of Katharina in 3 types of experimental plots1, namely, low, CONTROL, and high densities2, and comparing survivorship of Hedophyllum in each, the researchers find that Katharina affects survival of both juvenile3 (<1cm holdfast diameter) and young adult (1-4cm) plants, but not of adult (>4cm) plants. Note in the histogram that Hedophyllum densities decline in all treatment groups, including the CONTROLS over the 1-yr study period, in major part owing to winter-storm damage.  However, by April 1996 there is a significant difference between densities in the low and high treatment areas, and after this date there are no surviving Hedophyllum individuals in the high-density treatment areas. The effect on juvenile kelp plants is direct, in that the chitons eat them up. Effects on young adult plants, however, are indirect, in that the chitons damage the holdfasts resulting in greater losses to waves.  Apparently, the chitons seek out protection from desiccation or waves by burrowing into and edging under the holdfasts, thus loosening their attachment to the rock. The findings of the study provide evidence for a refuge in size for Hedophyllum from one of its principal grazers.  Markel & DeWreede 1998 Mar Ecol Progr Ser 166: 151. Photographs courtesy Russ Markel, University of British Columbia & Bamfield Marine Sciences Centre.photograph of chitons and kelp plants being counted

NOTE1 the authors do not use fence barriers to delineate these plots; rather, they employ natural topographical features such as surge channels and outcroppings to restrict the chitons’ movements

NOTE2 experimental densities are based on the original average density of Katharina in the 18 treatment areas, adjusted to 25% of this value in the “low-density” treatments and to 75% in the “high-density” treatments. There are 6 replicates of each of the 3 treatments

NOTE3 algal sporelings are highly vulnerable to grazing invertebrate herbivores and this early phase of life has been termed a “herbivory bottleneck” by marine algologists.  Although the same principles applies to larvae of marine invertebrates, there seems to be no comparable “carnivory bottleneck” referred to in the literature

 
Research study 5
 

map showing site locations for study on chitons Katharina tunicatagraph showng relationship of chiton density to species richnessHow does one go about selecting the best site to designate as a “no-take” reserve for black-leather chitons Katharina tunicata, or for any species for that matter.  Researchers at Bamfield Marine Sciences Centre, British Columbia start by selecting 10 rocky intertidal sites in Barkley Sound where this chiton species lives along with its principal food, the brown alga Hedophyllum sessile (see map).  They then assign a “conservation” value to each site based upon 5 metrics, 3 of which address representation (habitat type, species richness, species diversity), and 2 of which reflect fitness characters of the species in question (production and viability).  Interestingly, across all sites, general species richness is negatively correlated with chiton density (see graph).  Thus, where there is high diversity of species, fewer chitons contribute proportionally less to potential reproductive output, defined by total gonad mass.  A site chosen to be a reserve on the basis of species richness, then,  would actually not favour chiton vitality.  Note also on the graph that wave exposure is correlated neither with chiton density nor species richness.  Overall, the study shows that a site’s conservation value differs according to which metric is considered.  If true for other species, this finding will add an unexpected degree of complexity to the decision-making process relating to establishment of  “no-take” reserves.  Solomon et al. 2006 Biol Conserv 128: 79.

NOTE  the 10 sites are later divided into 5 wave-exposed and  5 sheltered ones

 
Research study 6
 

histogram showing age-frequency distribution for sample population of chitons Cryptochiton stelleri at Cape Blanco, OregonThe gumboot chiton Cryptochiton stelleri is the largest chiton in the world, yet little is known of its population dynamics or ecology.  An investigation of C. stelleri’s distribution at 6 rocky intertidal sites by a researcher at the Oregon Institute of Marine Biology, Charleston helps to redress the situation.  Results show highly patched and clumped distributions, mostly within wave-protected coves.  The preference for wave-protected areas may in part owe to a weaker attachment tenacity than other wave-tolerant chiton species (e.g., black leather chitons Katharina tunicata), but probably more to a greater abundance in such habitats of thin-bladed algal species such as Mazzaella splendens and Cryptopleura sp. favoured as food by C. stelleri.  Age-frequency plots reveal that recruitment is sporadic, low, and site-specific (see representative plot below for Cape Blanco).  Juveniles less than 15cm in length are rare, and when present tend to be found within pits in sandstone created by purple sea urchins.  The author suggests that the slower, swirling water-conditions in cove habitats may favour the trapping and settlement of larvae1, and later survival of recruits. The age-frequency data presented in the histogram are based on growth lines in shell plates, and indicate that maximum age is about 40yr.  Results from monitoring of tagged2 individuals show that C. stelleri are homebodies, and move less than about 20m on average over a 6mo period.  The author notes a lack of evidence of age-related movements of individuals within the vertical range of intertidal zone.  Laboratory experiments in which 10-30cm-sized individuals are maintained together with sea-stars and crabs in aquariums for 2mo reveal no predation3.  The author suggests that population-connectivity between sites on the Oregon coast is low, but notes that the necessary genetic work has not yet been done to show this.  The study is a welcome contribution to an under-investigated group of intertidal herbivores.  Lord 2011 Malacologia 54 (1-2): 147. Photograph photograph of gumboot chiton Cryptochiton stelleri courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washingtoncourtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington.

NOTE1  larvae are lecithotrophic and are able to settle within a few days of hatching

NOTE2  individuals are tagged by inserting plastic zip ties through the mantle edge bearing numbered fish tags.  Tag retention is poor, with only 14% being found after 6mo

NOTE3  predatory species tested include sea stars Evasterias troschelii, Pisaster ochraceus, and Pycnopodia helianthoides, and crabs Pugettia producta and Cancer productus

 

Gumboot chiton Cryptochiton stelleri
partially submerged in a tidepool 0.5X

 
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